The present invention relates to an autothermic reforming reactor.
During the operation of fuel cells having polymer membranes, briefly called PEM fuel cells, in particular for mobile applications, it is possible to produce a hydrogen-rich gas from a liquid raw fuel by reforming liquid hydrocarbons, for example, gasoline or Diesel fuel.
This reaction can advantageously be carried out in an autothermic reactor. There inside, energy is released by an exothermic combustion which is used for heating the endothermic reforming reaction. In the ideal case, the exothermic reaction zone is superimposed over the endothermic reaction zone. However, it is also possible for the exothermic reaction to be arranged upstream of the endothermic reaction. In the endothermic reaction zone, the charged water/air/hydrocarbon mixture is converted to an H2-rich gas which also contains CO in addition to CO2. To prevent the CO gas in the gas mixture from reacting back to elemental carbon (soot) at the end of the reactor, it is required for the gas mixture to be quickly cooled down to a low temperature level. This is achieved by adding water and is referred to as quenching. For the case of the partial oxidation of hydrocarbons, this process is described, for example, in U.S. Pat. No. 5,358,696 or in U.S. Pat. No. 2,664,402.
Due to the quenching process, a temperature gradient which corresponds to the cool down arises in the reactor. This is undesirable because the accompanying heat loss produces too low a temperature in the rear region of the endothermic zone. This temperature is decisive for the gas composition since the thermodynamic equilibrium of the reforming reaction is temperature dependent.
German Patent Application DE 197 11 044 A1 describes a reactor for sewage sludge incineration. It is aligned vertically, the combustion zone for incinerating the sewage sludge being situated in the lower region, and the combustion gases, which rise within the reactor, being used for drying the sewage sludge, which enters the reactor from above. Located between the drying zone and the combustion zone is a solid-state radiator having radially arranged, inclined blades similar to a fan wheel. The blades are inclined by 30xc2x0 to the flow direction of the combustion gases. When passing through the solid-state radiator, the combustion gases give off part of their heat to the solid-state radiator. A part of the absorbed energy is reflected back into the combustion zone as solid-state radiation.
An object of the present invention is to optimize the temperature distribution in the autothermic reactor in such a manner that the reaction zones are thermally decoupled from the quench zone in the best possible manner. A cool down in the rear region of the endothermic zone should be prevented; however, as good as possible an energy feedback into the upstream endothermic zone should be achieved. In this context, the arising loss of pressure of the gas volume flow should be as low as possible.
This objective is achieved by the autothermic reactor according to Patent Claim 1. Advantageous embodiments of the present invention are the subject matter of subclaims.
a thermal insulation for thermally insulating the endothermic reaction zone and the quench zone,
a thermal radiator for radiating the thermal energy absorbed from the effluent reactor gas volume flow. Its surface faces the endothermic reaction zone. The radiant power increases with the fourth power of the surface temperature in accordance with the Stefan-Boltzmann law. The hotter the gas temperature, the hotter is the surface of the heat shield and the higher is the energy radiated in the direction of the endothermic reaction zone.
Thus, mainly the following heat transfer mechanisms are of importance for the mode of operation of the temperature-stabilized reactor according to the present invention:
heat transfer from the gas volume flow to the heat shield; here, above all, the convective heat transfer through forced convection is of importance. In this connection, it is advantageous to produce a turbulent flow. This turbulent flow can be achieved by a suitable geometric design of the heat shield. Moreover, the heat shield geometry can be designed in such a manner that the heat flow to the surface of the thermal radiator is optimized.
radiant heat transfer from the heat shield back to the reforming catalyst located in the reaction zone. Typical temperatures during the execution of the reforming reaction with gasoline or Diesel fuel lie in the range of about 900xc2x0 C. At these temperatures, the radiated power is already relatively high. Via the surface type (ideally having the characteristic of a blackbody radiator) as well as the surface quality, it is possible to attain maximum radiation efficiency.
Thus, a very good thermal insulation of the quench zone from the autothermic region of the reactor is achieved using the heat shield according to the present invention. The endothermic reaction zone can be maintained at operating temperature without being influenced by the temperature drop in the quench zone.
At the same time, an energy feedback is attained in that the heat of the gas volume flow which has been absorbed by the heat shield is reflected back into the endothermic zone. Therefore, the heat losses caused by the effluent gas mass flow can be considerably reduced.
Moreover, the heat shield according to the present invention, advantageously acts as an energy store during load changes, as will be explained in the following:
In a reactor, one can essentially distinguish between two types of heat losses:
wall losses: heat losses to the surroudings through the reactor insulation and via the reactor surface;
gas volume flow losses: heat losses caused by the effluent gas mass flow and the gas temperature. The gas volume flow losses are dependent on the gas mass flow, the heat-absorption capacity of the flowing gas mixture, and on the temperature gradient.
During load changes, the exothermic energy release varies in proportion to the load change. Since the wall losses depend substantially on the inside temperature but the gas volume flow losses vary in proportion to the load, the proportions of the types of loss shift accordingly during load change. The lower the load, the higher is the proportion of the wall losses, and the higher the load, the higher is the proportion of the volume flow losses.
Under the described conditions, the beat shield according to the present invention can be used as a heat buffer for stabilizing the operating temperature of the reactor. During a downward change in load, it releases its previously absorbed energy in a time-delayed manner, depending on its heat-absorption capacity, thus slowing down the cooling. During an upward change in load, the proportionally increasing volume flow loss is reduced through the heat absorption of the heat shield.
Moreover, a more constant control of the quenching is attained via this damping process. This also permits a more uniform supply to the shift reaction for removing CO arranged downstream of a reactor.
The reforming catalyst according to the present invention can be used, in particular, for reforming hydrocarbons such as gasoline or Diesel fuel.
The reforming reactor according to the present invention can be used, in particular, in a fuel-cell powered motor vehicle for supplying the fuel cell with hydrogen.